I don't know how to get my NPCs into their house if it is above another NPC's house. I don't exactly know if they can jump on to wooden planks but it doesn't seem like they can. I want them to be able to go inside their houses without me making stairs every night.
This is a known bug with the last PC version of Terraria (version 1.1.2). All NPCs (both friendly and enemy) are unable to use wooden platforms when pathfinding. They will never jump up on them; friendly NPCs will also never fall down through them.
You can abuse this by putting NPC rooms in your town only on non-ground floors, with wooden platforms as the only way to get from one floor to another. After you've built the rooms this way, spend a full (in-game) night far away from your house, and the NPCs will all teleport to their rooms. Since they can't use the platforms to change floors, and the floors they're on have no exit to the outside, they'll never go outside to be killed by slimes again. Very convenient!
Here is an easy to have NPC's go down to your main room and back to their own rooms. They can just get anywhere, up and down. No switches, electronics, mods or hoists needed. Only woodplatforms and a building. Wanted to share the way I did it because I could not find the near-perfect way without switches or hoist.
I opened all doors so they move faster. They open it less often on their own. But this works regardless of open or closed doors.
They can walk on them but I've seen them jump onto just a platform. They only jump onto solid blocks. So no they can't.
Also, they will just teleport to their home if you wait long enough. You could also kill the NPC with lava if they get stuck or something. Then they should re-spawn in their home.
It seems like that they can't enter the houses, but if you wait long enough after they arrive, they will just teleport to their homes no matter where. There is something to consider tho. İf you are making houses on top of each other, DON'T make their walls shared. Don't use one houses ceiling as the other's floor.